Drug abuse and aggression are major, intertwined behavioral disturbances that constitute important and challenging public health and social problems. Antisocial personality disorder (APD) is one of the strongest psychiatric risk factors for drug abuse and aggressive behavior, making it an important etiologic and treatment target. In this new research project, we are testing hypotheses about how the presence of APD (and other comorbid psychiatric disturbances) might influence or be associated with aggressivity, risk-taking, emotional deficits, and serotonergic function. Concurrently, we also are testing how methadone pharmacokinetics might be related to these same parameters. In brief, we have developed behavioral indices of aggressivity and risk-taking as elicited by computer simulation in which the subject can earn points for money, but which also involves a provocation-induced "point-stealing" maneuver. Conjointly, risk-taking can be assessed within the framework of a more elaborated behavioral contingency. Emotional deficit is assessed in relation to startle eye blink reflex in response to acoustic and visual stimuli. Serotonergic function is evaluated in relation to prolactin response to fenfluramine challenge. Participants are methadone-maintained patients with established histories of opioid dependence, classified in relation to presence or absence of APD and other psychiatric disorders. After recruitment and baseline assessments, they undergo repeated testing for evaluation of aggressivity, risk-taking, emotional deficits, and serotonergic function. Analyses of the study data will focus on sub-group differences, wherein sub-groups are pre-defined by psychiatric status. This project is expected to yield new and extremely useful information about the behavioral and neurophysiological profiles of methadone-maintained patients who vary in relation to APD. The project also will yield more refined laboratory-based measures of aggressivity, risk-taking, and emotional deficit, enlarging the array of assesssments well beyond those based on interview or questionnaire data alone.